Disclosed is a device for preparation of liposomes, comprises a reaction tank and an infusion unit. The reaction tank comprises a collector mounted in a predetermined position of the reaction tank; Two inlet ports are included: the first inlet port for infusing an aqueous solution; and the second inlet port for infusing an organic solution. The infusion unit can introduce a bioactive agent containing-aqueous solution into the reaction tank. The infusion unit comprises a filter connected to one end of the infusion unit and being adjacent to the collector. The method using the device comprises the steps of infusing an aqueous solution and an organic solution into the reaction tank of the device and thus forming an interface between the filter and the collector; infusing a bioactive agent containing-aqueous solution and being filtered by the filter, the bioactive agent is encapsulated to form a water-in-oil emulsion; the water-in-oil emulsion is passing through the aqueous solution and thus to form a water-in-oil-in-water double emulsion. Finally the removal of the organic phase of water-in-oil-in-water double emulsion enables the harvest a plurality of liposomes. It has advantages such that simple-used and automation production. Thus nano size or sub-micro size liposomes can be prepared with high encapsulation efficiency without sonicators or delicate microfluidic systems.